In recent years, the concept of environmental protection has prevailed, and this concept encourages electronic devices to use lead-free components. As a result, many of electronic components undergo the change in specifications in order to eliminate lead from the materials.
On the other hand, the lead can advantageously suppress the growth of acicular whisker of metal, e.g. tin. In a conventional electronic component, in particular, the lead prevents lead-wires extending outside the electronic component from generating whisker. The lead-wire free from lead and extending outsides the components thus tends to generate whisker. On top of that, the whisker flows out of the electronic component due to vibration or shock, the whisker drops on a circuit board of an electronic device and incurs a short-circuit, so that the electronic device possibly encounters a failure. To overcome this problem, various measures are taken to prevent the generation, growth, and flow-out of the whisker.
FIG. 4 is a sectional view of an aluminum-electrolytic capacitor of surface mounting type. This capacitor is taken as an example of a conventional electronic component. This capacitor includes capacitor element 6 working as a functional element, lead-wires 1, outer case 7A, sealing member 7C, and insulating terminal board 8. Lead-wire 1 is led out from capacitor element 6. Cylindrical outer case 7A with a bottom accommodates capacitor element 6. Sealing member 7C is provided with through-holes 7D through each which lead-wire 1 passes. Sealing member 7C is disposed at an opening of case 7A, and is compressed by drawn-processed section 7B which is formed by drawing an outer wall of case 7A. Sealing member 7C thus seals the opening of outer case 7A. Insulating terminal board 8 is disposed on the opening of case 7A, and has through-holes 8A through each which lead-wire 1 led out from sealing member 7C passes. Terminal board 8 also is provided with grooves 8B formed on the outer wall of board 8, and each groove 8B accommodates lead-wire 1 bent at approx. right angles after passing through hole 8A.
Lead-wire 1 is formed of leader electrode 2 made of aluminum wire, metal wire 3, resin film 5. Metal wire 3 is a CP wire plated with tin, and first end 3A thereof is welded to first end 2A of leader electrode 2. Second end 2B of leader electrode 2 is connected to capacitor element 6, and second end 2B of metal wire 3 is connected to a circuit pattern (not shown) on circuit board 9 with solder 9A.
Resin film 5 covers welded section 4 between first end 2A of leader electrode 2 and first end 3A of metal wire 3. Thermosetting resin adhesive, e.g. epoxy resin, is employed as the material for resin film 5.
After the welding of leader electrode 2 to metal wire 3 and before welded section 4 is covered with resin film 5, lead wire 1 in process is washed by alkaline cleaning fluid, and then the alkaline cleaning fluid is removed. On top of that, lead-wire 1 is heated for approx. 21 minutes at 150° C. after or before the washing and the removal of the cleaning fluid.
The wash of welded section 4 with the alkaline cleaning fluid allows solving and removing tin attached onto the surface of welded section 4, so that the wash can prevent the generation of tin-whisker. The heat treatment at 150° C. allows easing the stress accumulated in grain boundary of aluminum, thereby preventing the tin in the grain boundary from forming acicular whisker or preventing the whisker from growing. Additionally, the form of resin film 5 prevents moisture, which causes the generation of whisker, from entering from the outside. As a result, the generation of the tin whisker can be prevented. Even if the whisker is generated, resin film 5 can block the outflow of whisker to the outside. The structure discussed above allows preventing tin acicular whisker from occurring at welded section 4 where aluminum is mixed with tin. (This is disclosed in, e.g. Patent Literature 1.)
Removal of tin from the surface of welded section 4 actually delays the generation of whisker some degree; however, a long-term environmental load test reveals the tin survived the washing and remained in depth direction of welded section 4, thereby still generating the whisker.
Although welded section 4 of lead wire 1 is coated with resin film 5, some material of film 5 cannot sufficiently prevent the generation of whisker. In this case, the whisker protrudes from welded section 4, and if vibrations or shocks are applied, the whisker flows outside the electronic component. As a result, the electronic device employing the foregoing conventional electronic component might encounter a short circuit or a malfunction caused by the acicular whisker dropped onto circuit board 9.